Glass-ceramic products have been developed a lot over the last years. They are commonly used notably as cook-tops, fireplace fire-window plates, pan-type cooking utensils, and the like. Such products are obtained by ceramming a glass precursor (called “green glass”). Products containing β-quartz solid solution as the main crystalline phase are usually transparent. Products containing β-spodumene solid solution as the main crystalline phase are usually translucent or opaque. Furthermore, the glass-ceramic can contain colorants within it, giving it a specific color.
A technique known in the prior art of coloring glass-ceramic product involves adding coloring metal ions into the precursor glass compositions. This is a coloration in the mass of the products which incorporates as ions of, for example, cobalt, chromium, iron, manganese, nickel, vanadium and/or cerium. This coloration technique is, on an industrial scale, a little arduous to manage because, in order to proceed form one batch having one color to another batch having another color, the manufacture process usually has to be interrupted. Furthermore, such a coloration in the mass is not able to confer a metallic appearance to the surface.
According to the prior art, other methods are also proposed for coloring and decorating glass-ceramic products.
WO-A-03 048062 and WO-A-03 048063 describe two “analogous” methods for coloring glass-ceramic products. These two methods are carried out on the finished product and consist in introducing coloring ions onto the surface of said product. These ions are introduced in the vapor phase, in the form of CuCl, CoCl2, NiCl2, MnCl2 or mixtures thereof (WO-A-03 048062), or dissolved in a solvent, in the form of oxides or chlorides of metals selected from Co, Ni, Mn, Cr, Fe and mixtures thereof (WO-A-03 048063). These coloring ions penetrate the surface of the glass-ceramic and diffuse into within it. They do not modify the crystalline structure of said glass-ceramic, and do not confer a metallic appearance to the surface of the glass-ceramic in question.
In order to attempt to confer such a metallic appearance to glass-ceramic plate surfaces, decorating enamel has been conventionally deposited on said surfaces. This is a surface treatment which is implemented on the finished product. The result is not fully satisfactory with reference to the metallic appearance sought after.
Another method has been proposed in US patent U.S. Pat. No. 3,926,602 with the same aim of conferring a metallic appearance to the surface of a glass-ceramic article. The glass precursor composition contains ferrous oxide (FeO) and, during the ceramizing heat treatment, the iron migrates to the surface and generates a surface layer of hematite (α-Fe2O3) thereon. Said heat treatment is somewhat delicate to implement. Apart from the ceramization, it must ensure the control of the migration or diffusion of the iron within the mass of the product undergoing ceramization.
In such a context, the inventors searched for a novel surface treatment method to decorate and/or color the surface of the glass-ceramic products, which enables novel glass-ceramic products to be made. Such method should overcome the disadvantages of the prior art methods described above, produce interesting effects on the appearance, yet does not compromise the mechanical properties of the material, and does not lead to undesirable alteration of the intrinsic properties of the glass-ceramic, such as ability to be scored or marked, and the like.
U.S. Pat. No. 3,764,444 describes glass-ceramic article having a surface layer with higher degree of crystallization than the body within, thus a higher flexural strength than an article containing only the same glass-ceramic as that of the main body portion. However, this reference does not disclose whether the surface modification would lead to change in appearance. In addition, the ion exchange process disclosed in this reference to achieve higher degree of surface crystallization is inconvenient to carry out.
U.S. Pat. No. 3,779,856 discloses a process for making glass-ceramic article having higher degree of surface crystallization involving nucleating the precursor glass in the presence of a reducing atmosphere. It is alleged in this reference that nucleating agents reduced near the surface area leads to a higher degree of crystallization. Again, this reference does not disclose whether the article thus produced would have changed appearance. In addition, the reference hints that this process may not be applicable for all lithium aluminosilicate-based glass-ceramic articles. In the claims of this reference, it is required that the precursor glass comprises CuO and/or Fe2O3. Moreover, the requirement of the presence of reducing atmosphere in the high temperature nucleating process renders it undesirable.
U.S. Pat. No. 3,615,318 discloses a process for decorating transparent glass-ceramic articles involving selective heating part of the surface to impart opacification thereof. The process requires the use of a high power specialty irradiation source, such as infrared lamp, and the selective application of irradiation absorbing materials onto the surface.
Japanese patent application publication Nos. 58185455 and 59035032 disclose that the surfaces of lithium aluminosilicate glass-ceramic articles may be modified by heat-treatment with a coating of crystallization-promotion agents applied. The crystallization-promoting agents include γ-Al2O3 and surfactant, Na2SO4 and Na2S2O7. While both references disclose that the flexural strength of the articles improved as a result, it is not clear whether the appearance of the article is changed.
In view of the prior art described above, there exists a genuine need of glass-ceramic articles with improved strength, appearance and ease of production. The present inventors have invented a new process for modifying the surface of lithium aluminosilicate-based glass-ceramic articles to produce glass-ceramic articles satisfying such needs.